Don't panic. This is not an uncommon event. Simply re-start your browser each time it happens. If it happens multiple times in one day, then try restarting your computer. If still issues, check your browser and Flash Plug-in are up to date.
There will be websites or more likely specific web pages that the Flash plug-in might struggle to handle. Keep that in mind.

Steve Jobs of Apple once said if there's anything that's ever going to crash on an Apple computer & do it at a consistent rate, it is the Adobe Flash plug-in.